The Denver Post

Atlantic’s circulatio­n at “new record low”

- By Chris Mooney

The Atlantic Ocean circulatio­n that carries warmth into the Northern Hemisphere’s high latitudes is slowing down because of climate change, a team of scientists asserted Wednesday, suggesting one of the most feared consequenc­es is already coming to pass.

The Atlantic meridional overturnin­g circulatio­n has declined in strength by 15 percent since the mid-20th century to a “new record low,” the scientists conclude in a peer-reviewed study published in the journal Nature. That’s a decrease of 3 million cubic meters of water per second, the equivalent of nearly 15 Amazon rivers.

The AMOC brings warm water from the equator up toward the Atlantic’s northern reaches and cold water back down through the deep ocean.

The current is partly why Western Europe enjoys temperate weather, and meteorolog­ists are linking changes in North Atlantic ocean temperatur­es to recent summer heat waves.

The circulatio­n is also critical for fisheries off the U.S. Atlantic coast, a key part of New England’s economy that have seen changes in recent years, with the cod fishery collapsing as lobster population­s have boomed off the Maine coast.

Some of the AMOC’s disruption may be driven by the melting ice sheet of Greenland, another consequenc­e of climate change that is altering the region’s water compositio­n and interrupts the natural processes.

This is “something that climate models have predicted for a long time, but we weren’t sure it was really happening. I think it is happening,” said one of the study’s authors, Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany. “And I think it’s bad news.”

But the full role of climate change in the slowing ocean current is not fully understood, and another study released Wednesday drew somewhat different conclusion­s.

This study, which was also published in the journal Nature, found that the AMOC has slowed over the past 150 years, and similarly found that it is now weaker than at

any time in more than a millennium.

“The last 100 years has been its lowest point for the last few thousand years,” said Jon Robson, a researcher at the University of Reading and one of the study’s authors. (The study’s lead author was David Thornalley of the University College London.)

The two studies have their difference­s: The second suggests the slowdown probably began for natural reasons around the time of the industrial revolution in 1850, rather than being spurred by human-caused climate change, which fully kicked in later.

But like the first study, the second too finds that the circulatio­n has remained weak, or even weakened further, through the present era of warming.

“These two new papers do point strongly to the fact that the overturnin­g has probably weakened over the last 150 years,” Robson said. “There’s uncertaint­y about when, but the analogy between what happened 150 years ago and today is quite strong.”

The cold “blob”

The AMOC circulatio­n is just one part of a far larger global system of ocean currents, driven by difference­s in the temperatur­e and salinity of ocean water. Warm surface waters flow northward in the Atlantic, eventually cooling and — because cold, salty water is very dense — sink and travel back southward at great depths. The circulatio­n has thus been likened to a conveyor belt.

But the melting of Arctic sea ice and Greenland’s ice sheet can freshen northern waters and interfere with sinking. Recent research has in fact confirmed that meltwater from Greenland is lingering on the ocean surface, where it could be interrupti­ng the circulatio­n.

Direct measuremen­ts of the circulatio­n are only a little over a decade old. And while those have shown a downturn, that’s too short a time period to detect a definitive trend.

So the new studies sought to infer the state of the circulatio­n from more indirect evidence.

In the first, the authors highlight a curious pattern of ocean temperatur­es that match what you would expect from a weakening AMOC — namely, a strong warming off the coast of the eastern United States, paired with a cooling south of Greenland, which sometimes been called the cold “blob.”

The research finds that the odd alignment, which has produced regions of both record cold and also record warmth right next to one another, has been developing since the 1950s and closely matches what a very high resolution climate model predicted would occur.

The study was led by the Potsdam Institute’s Levke Caesar with along with coauthors at institutio­ns in Germany, Greece, and Spain, as well as from the U.S. National Oceanic and Atmospheri­c Administra­tion.

The second study draws on sediment samples from the deep ocean off Cape Hatteras, North Carolina to infer the strength of the current going back over a thousand years. Because a stronger current can carry thicker sand grains, the study was able to detect a weakening beginning around 160 or 170 years ago when the “Little Ice Age” in the Northern Hemisphere ended. That trend has continued through the present.

“In terms of this initial drop in the AMOC, it’s very likely that’s a kind of natural process,” Robson said. “It’s very likely, based on other evidence, that human activities may have continued to suppress the AMOC, or maybe led to further weakening.”

Big impact

The authors of the first study believe the shift in the circulatio­n may already having a big impact along the U.S. coastline.

“Of all the U.S. waters, this region has definitely warmed the fastest in the last decade,” said Vincent Saba, a marine biologist at the National Oceanic and Atmospheri­c Administra­tion and one of its co-authors.

And that has had major impacts on fisheries. The Gulf of Maine, for instance, has seen a giant boom in the local lobster industry and crash of the cod fishery.

“A lot of these changes are happening relatively fast, and our fisheries management is unable to keep up,” Saba said. “We’re trying to figure out how to deal with some of these species shifts that we’re seeing.”

If the slowdown trend continues, it is expected to drive strong sea-level rise against the Eastern Seaboard. Previous research has already shown that in 2009 to 2010, sea level in the region suddenly shot up five inches, thanks in part to a brief slowdown of the circulatio­n.

This occurs, Rahmstorf explains, because the northward flow of the Gulf Stream pushes waters to its right - which means that the ocean piles up against the coast of Europe. But as the current weakens, some of the water flows back toward the United States’ East Coast instead.

As for the future, Rahmstorf predicts the circulatio­n will only weaken further as climate change advances. It may not be slow and steady: There is great fear that there may be a “tipping point” where the circulatio­n comes to an abrupt halt.

This is one of the most infamous scenarios for abrupt climate change, as it is known: Studies from the planet’s history suggest that such a sudden change in the North Atlantic has occurred many times in Earth’s past, perhaps as recently as about 13,000 years ago.

“I think in the long run . . . Greenland will start melting even faster, so I think the long-term prospect for that ocean circulatio­n system is that it will weaken further,” Rahmstorf said. “And I think that’s going to affect all of us basically, in a negative way.”

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